The present invention relates to a thin ribbon of a rare earth-based permanent magnet alloy, referred to simply as a thin alloy ribbon hereinafter, or more particularly, to a thin alloy ribbon which can be an intermediate material for the preparation of a sintered permanent magnet of the R/T/B-type or, in particular, neodymium/iron/boron-type having excellent magnetic properties.
The application fields of permanent magnets are rapidly growing and expanding in recent years in a great variety of electric and electronic industries as a very important key component in household electric appliances, computers and communication instruments as well as marginal instruments thereof, medical diagnostic instruments and so on. In compliance with the recent trend of computers and communication instruments toward compactness in design and lightness in weight and requirements for environment preservation and energy saving, extensive investigations are now under way for the development of permanent magnets of higher and higher performance.
Among a variety of permanent magnet alloys thus far developed and now under practical use, the so-called R/T/B-type magnet alloys or, typically, the neodymium/iron/boron-type magnet alloys are highlighted in respects of their high magnetic properties and relatively low material costs among the rare earth-based permanent magnet alloys. The magnet alloys of this type are prepared usually by the metal-mold casting method or by the strip casting method.
In the above mentioned metal-mold casting method, the magnet alloy is obtained in the form of an ingot by casting a melt of the alloy into a metal-made casting mold. This method is advantageous because a magnet alloy of an exactly controlled chemical composition can readily be obtained so that this method is widely employed. The metal-mold casting method, however, has a serious problem due to the relatively low rate of heat transfer between the casting mold and the alloy melt cast therein and within the magnet alloy per se necessitating a relatively long time for solidification of the molten alloy resulting in precipitation of the .gamma.-iron phase as the primary crystals in the course of solidification of the molten alloy with remaining .alpha.-iron phase of a coarse grain size of 10 .mu.m or larger in the core portion of the ingot after solidification by cooling. In addition, the grain size of the R-rich phase and R.sub.x T.sub.4 B.sub.4 phase surrounding the R.sub.2 T.sub.14 B phase also cannot be fine enough.
Since the cooling rate of the alloy is substantially different between the surface layer of the ingot adjacent to the walls of the casting mold and the core portion of the ingot, in addition, the .alpha.-iron phase and R-rich phase as precipitated may have a wide variation in the grain diameters so that the particle size distribution of the alloy particles prepared by pulverization of the alloy ingot is broad due to the difficulties encountered in the pulverization of the ingot into fine particles of a particle diameter of a few micrometers. Accordingly, the magnet alloy particles are inferior in the behavior for magnetic orientation and sintering resulting in a decrease in the magnetic properties of the sintered permanent magnets as the final products.
In the strip casting method, on the other hand, a melt of the magnet alloy is continuously ejected at the surface of a rotating quenching roller of the single-roller type or twin-roller type so that the alloy melt is rapidly solidified on the roller surface in the form of a thin ribbon of the alloy having a thickness of 0.01 to 5 mm. This method is advantageous because of the possibility of controlling precipitation of the .alpha.-iron phase and accomplishing fine and uniform dispersion of the R.sub.x T.sub.4 B.sub.4 phase by adequately selecting the quenching conditions of the alloy melt consequently leading to a uniform structure of the magnet to obtain high performance R/T/B-type permanent magnets.
Extensive investigations have been conducted on the metallographic structure of the thin alloy ribbon obtained by the strip carting method, sometimes referred to as a strip cast, with an object to upgrade the permanent magnets prepared from the thin alloy ribbons. For example, Japanese Patent No. 2639609, directing attention to the precipitation type of the .alpha.-iron phase in and the metallographic structure of the strip casts, discloses a thin alloy ribbon having a structure in which .alpha.-iron grains having a grain diameter smaller than 10 .mu.m are dispersed as the peritectic nuclei in the crystalline grains of the principal phase. Japanese Patent Kokai 7-176414 proposes a thin alloy ribbon having a structure substantially free from segregation of the .alpha.-iron phase. Further, Japanese Patent Kokai 10-317110 proposes, directing attention to the fine chill crystalline structure as formed in the vicinity of the solidification front, a rare earth-based magnet alloy for a base material of sintered permanent magnets.
Nevertheless, almost no investigations have been undertaken, in relation to the thin alloy ribbons obtained by the strip casting method directing attention to the four-phase coexisting region consisting of the .alpha.-iron phase, R-rich phase, R.sub.x T.sub.4 B.sub.4 phase and R.sub.2 T.sub.14 B phase as well as the chill crystals formed on and in the vicinity of the solidification front, on the correlation of the magnetic properties with the precipitation type and structure thereof.